Insert new content — one or more blocks, or prose — at the end of the document (position: append) or before/after the block named by anchor. Use this for content that does not exist yet; to change a block that does, use geml_set. Ids inside the content are kept. In a .md file Markdown lands as written, and GEML content is converted to Markdown as to: "md" converts it, the result's notes saying so; content Markdown cannot hold, such as a view without its source, is refused. Like every write here it is validated first: a missing anchor, an id that clashes with an existing one, or content that would break the document is refused. Returns {ok, file, diagnostics, revision}, with notes when the write did something to say out loud (a block it dropped, an address it changed); a refusal is ok: false with a hint, and the file is unchanged. This server is STATELESS: the document travels in source and a write comes back as document for you to save; it holds no other documents, so a cross-document reference such as [[other.geml#id]] cannot be resolved here and is reported as unresolvable-document.